1. Field of the Invention
The present invention relates to a process for the preparation of photographic coupler dispersions. More particularly, it relates to a process for removing a partially water-soluble auxiliary solvent contained in a photographic coupler dispersion in producing the photographic coupler dispersion.
2. Description of the Prior Art
A photographic coupler dispersion has hitherto been produced by dissolving a coupler in an appropriate high-boiling point solvent and homogenizing the resulting solution in an aqueous hydrophilic colloid, e.g., gelatin solution. To accomplish this procedure, however, an excess amount of the high-boiling solvent in comparision with the amount of the coupler must be used. Such an excess amount of the high-boiling solvent does not contribute at all to the photographic properties, and it decreases the physical hardness of the coating film. Therefore, a procedure has generally been employed which comprises adding a water-insoluble or water-soluble auxiliary solvent having a lower boiling point to the solution prepared by dissolving the coupler in the high-boiling solvent, and then dispersing the resulting solution in an aqueous hydrophilic colloid, e.g., gelatin solution (for example, as disclosed in U.S. Pat. Nos. 2,801,170, 2,801,171, etc.).
Since such an auxiliary solvent-containing coupler dispersion has poor stability with the lapse of time, removal of such an auxiliary solvent is desirable.
Known procedures for removing such an auxiliary solvent include a method in which a coupler dispersion is gelled by cooling, the gel is cut into small pieces or extruded in a noodle form, and the gel thus obtained is washed with cold water, a method in which a coupler dispersion is extruded from a heat-insulating extrusion head into cold water to form a noodle of the dispersion instantaneously, and the product so obtained is washed with cold water (e.g., as disclosed in U.S. Pat. No. 3,396,027), a method in which a noodle of a coupler dispersion is dried with air, a method in which a coupler dispersion is coated on a drum in the form of a thin film and dried, etc., are known.
The procedure in which the dispersion is gelled, cut into small pieces or extruded in a noodle form, and washed with water, however, has the disadvantages that the gel absorbs water during the processing, swells and disintegrates, that the gel dissolves in the washing water, and that the water content of the coupler dispersion after the processing is increased more than necessary.
In the procedure in which the auxiliary solvent is removed by evaporation in the air or in vacuo, vaporization of the water together with the auxiliary solvent and increase of the water content of the coupler dispersion more than necessary are problems.
Such a change in the water content is a quite serious problem from the standpoint of process control, and it is also an important problem from the standpoint of storage stability of the dispersion.